A fuel cell, which directly converts chemical energy generated by oxidation into electrical energy, is a new green futuristic energy technology which can generates the electrical energy from materials such as oxygen, hydrogen and the like which is found in abundance on the earth.
In the fuel cell, oxygen is supplied to a cathode and hydrogen is supplied to an anode so that an electrochemical reaction is performed in a reverse way of water electrolysis so as to generate electricity, head and water, thereby producing the electrical energy without any contamination.
Since the fuel cell is free from limitation of Carnot cycle efficiency which acts as the limitation in a conventional heat engine, it is possible to increase an efficiency of 40% or more. Further, since only the water is exhausted as emissions, there is not a risk of environmental pollution. Furthermore, since there is not a necessity of a place for mechanical motion, unlike in the conventional heat engine, it has some advantages of reducing a size and a noise. Therefore, the fuel cell technologies (e.g. material, fabrication, etc) are actively investigated at many famous laboratories all over the world at present.
According to a kind of electrolyte used therein, the fuel cell is classified into a PAFC (Phosphoric Acid Fuel Cell), a MCFC (Molten Carbonate Fuel Cell), a SOFC (Solid Oxide Fuel Cell), a PEMFC (Polymer Electrolyte Membrane Fuel Cell), a DMFC (Direct Methanol Fuel Cell) and an AFC (Alkaline Fuel Cell) which are already being used or developed. Characteristics thereof will be described in a table.
PAFCMCFCSOFCPEMFCDMFCAFCElectrolytePhosphoricLithiumZirconia/HydrogenHydrogenPotassiumacidcarbonate/CeriaionionhydroxidePotassiumseriesexchangeexchangecarbonatemembranemembraneIonHydrogen ionCarbonicOxygenHydrogenHydrogenHydrogenconductoracid ionionionionionOperation200650500~1000<100<100<100temperatureFuelHydrogenHydrogen,Hydrogen,HydrogenmethanolHydrogencarbonhydrocarbon,monoxidecarbonmonoxideRawCity gas,City gas,City gas,Methanol,methanolHydrogenmaterial ofLPGLPG, coalLPG,MethanefuelHydrogengasoline,HydrogenEfficiency 40 45 45   45   30   40(%)Range of100-50001000-1000000100-1000001-100001-1001-100outputpower (W)ApplicationDistributedLargeSmall,PowerPortablePowerpowerscalemiddlesourcepowersource forgenerationpowerand largeforsourcespace shipgenerationscaletransportpowergenerationDevelopmentDemonstrated-Tested-Tested-Tested-Tested-Applied tolevelutilizeddemonstrateddemonstrateddemonstrateddemonstratedspace ship
As described in the table, the fuel cells have various ranges of output power and applications and the like. Thus, a user can selectively use one of the fuel cells for various purposes. Particularly, the SOFC has a disadvantage that its operation temperature is high, but also has an advantage that it can be used for large scale power generation.
In an operation principle of the SOFC, oxygen is supplied to the cathode and hydrogen is supplied to an anode. At this time, the reaction is performed as follows:
Reaction in the anode2H2+2O2−→2H2O+4e−
Reaction in the cathodeO2+4e−→2O2−
In the SOFC having the characteristic described above, the higher the diffusion performance of the hydrogen supplied to the anode is, the more an efficiency of the fuel cell is increased. Therefore, in order to increase the diffusion performance of the hydrogen supplied to the anode, a gas diffusion layer is formed by artificially adding an additive like polymer or carbon.
In case that the conventional SOFC has a gas diffusion part, strength of the SOFC is reduced. But if a thickness of the anode is increased in order to solve the problem, gas diffusion is deteriorated, and thus a performance of the fuel cell is also deteriorated. Particularly, the performance of the fuel cell is damaged in a high current range.
Further, according as the reaction is processed, a gas diffusion path is clogged by carbon deposition. Thus, fuel supplying to a catalytic layer contacted with an electrolyte layer is blocked. Since it makes difficult to collect current generated from the anode, there is a problem of electric power loss.
Meanwhile, in fabricating of the SOFC, a powder compressing method has a disadvantage that cracks are easily generated by a small pressure since a density in an entire area, thereby shortening a lifetime thereof and also deteriorating a fuel cell performance.